Flexographic printing plates are sometimes known as “relief printing plates” and are provided with raised relief images onto which ink is applied for application to the printing substance. Flexographic printing plates generally have a rubbery or elastomeric nature. Flexographic printing plates have been imaged in a number of ways. A common method of imaging the plate is to prepare the relief images by exposing photosensitive compositions coated on a substrate through a masking element or transparency and then removing non-exposed regions of the coating with a suitable solvent. The remaining exposed areas are not removed due to the crosslinking of the photosensitive composition that renders these areas insoluble in the wash (or developing) solvents. These remaining areas provide the image areas to be inked and constitute the working part of the flexographic printing plate. Flexographic printing can also be carried out using a flexographic printing cylinder or seamless sleeve having the desired raised relief image. With the recent availability of high power lasers, such as CO2 lasers and IR laser diodes, it is now possible to use such lasers to formulate flexographic printing plate precursors that can be directly engraved to provide the desired relief surface necessary for flexographic printing. This is known as “direct laser engraving” and provides advantages by eliminating the need for a negative or a photomask to make the flexographic printing plate and eliminating the use of solvent processing.
The non-printing wells in the relief of a flexographic printing plate are at least 0.05 mm in depth in the screen areas, and can assume values up to 3 mm in other imaged areas in the case of thick flexographic printing plates or other flexographic printing members. Thus, large amounts of material must be removed, for example, by means of the laser. Direct laser engraving therefore differs very substantially in this respect from other techniques known from the printing plate sector, in which lasers are used only for imaging thin layers such as for a lithographic printing plate or a mask that is used over a photopolymer flexographic plate, for which the actual production of the flexographic printing plate is still effected by means of a washout and development process.
Various vulcanized thermoplastic elastomeric materials and olefinic polymers have been described for use as binders in laser-engraveable layers in flexographic printing element precursors. For example, U.S. Pat. No. 6,776,095 (Telser et al.) describes using a two-step crosslinking process and various thermoplastic elastomer block copolymers to provide laser-engravable flexographic printing plate precursors. The thermoplastic elastomers can be styrene-butadiene and styrene-isoprene block copolymers that are used in combination with suitable crosslinking chemistry.
Particulate materials such as inorganic fillers and organic particles have been added to elastomeric compositions used in laser-engraveable (or laser-ablatable) compositions to improve engraveability. Voids can also be provided in such compositions for the same purpose. For example, U.S. Pat. No. 6,159,659 (Gelbart) describes a flexographic printing plate that has an elastomeric non-porous top layer and a porous laser ablatable layer under the top layer. The laser ablatable lower layer contains voids that can be created by incorporating glass particles or plastic micro-balloons for example commercially available Expancel® beads that have a size of about 50-100 μm.
Similarly, U.S. Pat. No. 6,090,529 (Gelbart) describes a structure in which the porous layer is a foamed layer or can contain micro-balloons, and further describes a method to create the recessed image areas by using a lower power laser that melts and shrinks the porous layer in specific areas rather than ablating it.
U.S. Pat. No. 6,989,220 (Kanga) describes a method of making an imaged relief plate by providing a collapsible curable layer containing microspheres. The microspheres can be either expanded or unexpanded microspheres having a thermoplastic shell containing a hydrocarbon liquid (for example, Expancel® beads) having sizes in the range of 6 μm to 40 μm.
U.S. Pat. No. 4,060,032 (Evans) describes a multilayered composite plate with low density that can be achieved by incorporating expandable micro-capsules made of a thermoplastic shell of poly(vinylidene chloride-co-acrylonitrile) and a nucleus of a liquefied blowing agent in specified polymer binders. The microcapsules can be expanded during flexographic printing plate manufacturing. The resulting foam cell size in the imageable layer ranges from 14 to 20 μm.
U.S. Patent Application Publication 2008/0261028 (Regan et al.) describes laser-ablatable compositions and printing plate precursors that include various elastomeric resins and inorganic or organic particles.
Copending and commonly assigned U.S. Ser. No. 13/053,700 (filed Mar. 22, 2011 by Landry-Coltrain and Franklin) describes laser-engraveable layer containing nanocrystalline polyolefins and optionally chemically inactive particles or microcapsules to reinforce the mechanical properties of the material, increase the hardness of the material, or decrease the tackiness of the material and ablation debris, enabling easier debris collection and improving the cleanliness of the laser-engraved member. Inactive inorganic particles include various inorganic filler materials, inactive microcapsules such as those described in the publications cited above. Inactive microspheres can be hollow or filled with an inactive solvent, and facilitate engraving in the laser-engraveable layer because they reduce the energy needed for engraving. Useful inactive microspheres can be formed of an inorganic glass material such as silicon oxide glass, a magnesium silicate glass, or an inactive thermoplastic polymeric shell material such as a styrene or acrylate copolymer or a vinylidene chloride copolymer.
Still other useful nanocrystalline polyolefins can be prepared using synthetic procedures as described for example in U.S. Pat. Nos. 6,930,152 Hashimoto et al.) and 7,253,234 (Mori et al.) and U.S. Patent Application Publication 2008/0220193 (Tohi).
A significant problem is encountered, however, by using known organic porous materials such as plastic micro-balloons, expandable microspheres, and microcapsules, in that they are all made of a thermoplastic shell that is not crosslinked. This property results in the deformation and collapse of the microcapsules and a consequent loss of their porosity during a manufacturing processing step typically used to fabricate flexographic plate precursors. Typical melt processing steps include melt mixing and melt extrusion at elevated temperatures. In addition, in manufacturing processes using organic solvent coating, the microcapsules would potentially dissolve in the organic solvent, resulting in a loss of porosity and thus restricting the choice of organic solvents that could be used to manufacture the flexographic printing plate precursors.
While the known laser-engraveable compositions can be used with the known particulate materials, there is a need to improve the particulate materials so that imaging speed is increased to increase printing precursor throughput, and to improve melt processability. These properties are not readily achieved using organic particles known in the art. It would also be useful to have organic porous particulate materials having two or more discrete pores into which various radiation absorbers can be incorporated if desired.